jclouds- how to uniquely identify one node within a group of compute instances

In a java app, I have started a group of virtual machines with a specific group name.

Now, I wish to identify each of the virtual machines in that group, uniquely. The code to identify a virtual machine (which is part of that group of machines) will be run within that machine itself directly. What parameter/attribute do I use to uniquely identify such a machine? As I understand "NodeMetadata" identifies each node...But which attribute from "NodeMetadata" class do I use for the purpose as described above? In such a way that, this attribute can be obtained by code running on the node that I wish to terminate?